There is disclosed an image forming apparatus, such as a laser printer, of a type that a developing cartridge is attached to the main body of the apparatus as to be detachable therefrom (See Japanese Unexamined Patent Application Publication No. 2006-267994). The developing cartridge contains a developer. When the developing cartridge runs out of the developer, the cartridge is removed from the main body of the apparatus. Then, a new developing cartridge is attached to the main body. Furthermore, when the apparatus cause a paper jam within the main body, the developing cartridge may be removed from the main body to eliminate such a paper jam, and then attached again to the main body.
In the image forming apparatus of this type, it is suggested how to determine whether the developing cartridge is a brand-new or used one when attached to the main body as a way to find out the wear of the developing cartridge.
On the side surface of such developing cartridge is a detecting gear mounted, and the detecting gear is rotatable around an axis line (rotation axis line) extending in a transverse direction crossing the side surface at a right angle. The detecting gear has a plate-shaped detecting gear body and a contact protrusion integrally formed with the detecting gear body on the outer side (the opposite surface to the side of the developing cartridge with respect to the detecting gear body) of the detecting gear body. The detecting gear body has gear teeth on its circumferential surface (except some portion of the circumferential surface).
Further, a transmission gear is provided on the side surface of the developing cartridge, and the transmission gear is rotatable around an axis line extending parallel to the axis line of the detecting gear at a distance. The transmission gear rotates as a whole with an agitator for agitating the developer contained in the developing cartridge. The transmission gear has gear teeth on its entire circumferential surface.
In a new developing cartridge, the gear teeth of the transmission gear are engaged with the gear teeth of the detecting gear. When the developing cartridge is attached to the main body, the driving force of a motor is delivered to the transmission gear, and further transmitted from the transmission gear to the detecting gear through those gear teeth.
This allows the detecting gear to rotate, and the contact protrusion to move in the rotational direction of the detecting gear in response to the rotation of the detecting gear. When the toothless portion of the detecting gear faces the gear teeth of the transmission gear, the gear teeth of the transmission gear is disengaged with the gear teeth of the detecting gear, and the rotation of the detecting gear stops. Thus, if the developing cartridge is ever attached to the main body, the gear teeth of the transmission gear is disengaged with the gear teeth of the detecting gear, and such position remains afterwards.
In the main body is a sensor mounted for detecting the penetration of the contact protrusion, given that the contact protrusion is a detected part. Then, based on the detection result as to the penetration of the contact protrusion by the sensor, an old or new developing cartridge is determined. In other words, after a developing cartridge is attached to the main body, the developing cartridge is determined new if the sensor detects the penetration of the contact protrusion. On the other hands, after a developing cartridge is attached to the main body, the developing cartridge is determined old if the sensor does not detect the penetration of the contact protrusion.